1. Field of the Invention
The present invention relates to a DC--DC converter (forward converter) which comprises a MOSFET synchronous rectifying element, and is suitable for parallel operation.
2. Description of the Related Art
A circuit of a major part of a DC--DC converter (forward converter) comprising a synchronous rectifying element of a MOSFET is shown in FIG. 7. The circuit is disclosed in Japanese Unexamined Patent Publication No. 9-51260, and is a type where an input side circuit is isolated from an output side circuit by a transformer 10. In FIG. 7, one terminal of the primary coil 11 of the transformer 10 is connected to the anode of a DC input source 14, and the other terminal of the primary coil 11 is connected to the drain of a main switching element Q1 comprising a MOSFET. The source of the main switching element Q1 is connected to the cathode of the DC input source 14, and the gate of the main switching element Q1 is connected to a pulse width control circuit 8.
The secondary coil 12 of the transformer 10 is connected to a synchronous rectifier driving circuit 1. The synchronous rectifier driving circuit 1 comprises a synchronous rectifier 2 on a rectification side comprising a MOSFET, an input capacitor 4 on the rectification side, and a clamp diode 6 on the rectification side, and one terminal side of the input capacitor 4 on the rectification side is connected to one terminal side of the secondary coil 12, and the other terminal side of the input capacitor 4 on the rectification side is connected to the gate of the synchronous rectifier 2 on the rectification side. The drain of the synchronous rectifier 2 on the rectification side is connected with the other terminal of the secondary coil 12, and between the gate and the source of the synchronous rectifier 2 on the rectification side, the clamp diode 6 on the rectification side is connected so as to have its cathode side connected to the gate of the synchronous rectifier 2.
A connection of the secondary coil 12 to the input capacitor 4 on the rectification side is connected with an output terminal (+ side output terminal) 29, and the source terminal of the synchronous rectifier 2 on the rectification side is connected with an output terminal (- side output terminal) 30 via a conductor line 31. Between the conductor line 31 and an input terminal of a choke coil L, a diode D1 is connected such that its cathode is connected to the choke coil, a smoothing capacitor 16 is connected between the conductor line 31 and an output terminal of the choke coil L, and a load is connected between the output terminals 29 and 30. These connecting circuits of the secondary coil 12, the synchronous rectifier driving circuit 1, the diode D1, the choke coil L, and the smoothing capacitor 16 constitute a rectifying smoothing circuit 18.
A voltage detecting terminal for detecting an output voltage is connected to an output terminal side of the choke coil L, and an output voltage detected by the voltage detecting terminal is applied to a comparator circuit 9. The comparator circuit 9 compares the detected voltage applied from the voltage detecting terminal with a reference voltage, and a signal comprising the compared result is applied to a pulse width control circuit 8. The pulse width control circuit 8, receiving the signal from the comparator circuit 9, controls a pulse width of a switch driving control signal to be applied to the main switching element Q1 so as to have the output voltage at a fixed constant voltage.
When the main switching element Q1 is turned on in this circuit, the secondary coil 12 outputs a voltage of the primary coil 11 in a ratio (n2/n1) comprising the number n2 of turns of the secondary coil 12 relative to the number n1 of turns of the primary coil 11. At this time, a voltage is generated in a direction from the input capacitor 4 on the rectification side toward the gate of the synchronous rectifier 2 on the rectification side, an electric charge is charged on an input capacitance C.sub.iss of the capacitor 4 on the rectification side and the synchronous rectifier 2 on the rectification side, and the synchronous rectifier 2 on the rectification side is turned on. A voltage outputted from the secondary coil 12 is rectified by the synchronous rectifier 2 of the rectification side and the diode D1, then smoothed by the choke coil L and the smoothing capacitor 16, and supplied to a load as a DC output voltage V.sub.out in a substantially constant voltage. At this time, the diode D1 stays in an off-state.
When the main switching element Q1 is turned off, a voltage is generated at the secondary coil 12 in the opposite polarity as the voltage generated when the main switching element Q1 is turned on, and the diode D1 is turned on. In an on-period of the main switching element Q1 (on-period of the synchronous rectifier 2 on the rectification side), the electric charge charged on the input capacitance C.sub.iss of the input capacitor 4 on the rectification side and the synchronous rectifier 2 on the rectification side is discharged, and the synchronous rectifier 2 on the rectification side is turned off. On the other side, the synchronous rectifier 2 on the rectification side is turned on when a voltage V.sub.gs across the gate and source of the synchronous rectifier 2 on the rectification side is at -Vf (Vf: a forward direction voltage drop of the clamp diode 6 on the rectification side) to cause an electric current to flow, and the minimum value of the voltage V.sub.gs across the gate and source of the synchronous rectifier 2 on the rectification side is clamped at -Vf. Consequently, the voltage across the gate and source of the synchronous rectifier 2 on the rectification side during the on-period of the synchronous rectifier 2 on the rectification side is maintained unchanged at a constant level, despite a change of the duty of the main switching element Q1.
In other words, when the electrostatic capacitance of the input capacitor 4 on the rectification side is C2, input capacitance of the synchronous rectifier 2 on the rectification side is Ciss, and an output voltage of the secondary coil 12 is V2, at the time of steady operation, the voltage Vgs across the gate and source of the synchronous rectifier 2 on the rectification side at the time when the main switching element Q1 is on (when the synchronous rectifier 2 on the rectification side is on) is determined by the follwing equation, namely; EQU Vgs={C2/(Ciss+C2)}.times.V2
As can be understood from the equation, by setting a ratio for C.sub.iss relative to C2 at optimum, an optimum gate driving voltage of the synchronous rectifier 2 on the rectification side can be set, and as the optimum gate driving voltage can be maintained at the constant level irrespective of change of the duty of the main switching element Q1, by clamping action of the clamp diode 6 on the rectification side, there is an advantage that the gate driving loss of the synchronous rectifier 2 on the rectification side can be minimized.
Waveform A in FIG. 4 is a gate driving waveform of a synchronous rectifier 2 on the rectification side in a circuit of the above-described conventional embodiment, and as can be understood from the waveforms, at a turned-on point of a switch, a spike voltage S caused by the leakage inductance of the transformer 10 is generated, and is applied across the gate and source of the synchronous rectifier 2 on the rectification side, and across the cathode and anode of the Diode D1. As the gate driving loss due to the spike voltage S increases with the increase of the leakage inductance of the transformer 10, it is likely that breakdown of the synchronous rectifier 2 on the rectification side or the diode D1 may be caused. Therefore, improvement thereof is desired.
As shown in FIG. 8, as an application mode of a DC--DC converter (hereinafter also called as forward converter), a system is employed wherein a plurality of the forward converters (two forward converters in FIG. 8) are driven in parallel to supply a fixed DC current from respective forward converters to the common load. This sort of parallel operation is employed when an output current from one forward converter is not enough for the current quantity required by a load, or the like.
However, when such forward converters are operated in parallel, timing of circuit activating operation shifts because of variations in characteristics of circuit parts or the like of respective forward converters, and a phenomenon occurs, for example, while a forward converter A has started switching operation, a forward converter B is still in non-operating state. If such situation occurs, by an output voltage of the forward converter A which is in the operating state, a synchronous rectifier of the forward convert B is mistakenly turned on, and a current flows from the output terminal of the forward converter A to the output terminal of the forward converter B which is in non-operating state, and the current flows toward the secondary coil 12 side of the forward converter B in non-operating state, and thus a problem is caused that parts of the main switching elements Q1 or the like of the forward converter B are damaged by the reverse current.
When an output voltage of the forward converter A in operating state is applied across the gate and source of the synchronous rectifier 2 on the rectification side of the forward convert B and the threshold voltage of the synchronous rectifier 2 on the rectification side is exceeded thereby, a current flows in the reverse way from the output terminal of the forward converter B through the choke coil L of the forward converter B, and the secondary coil 12 of the transformer 10, to the synchronous rectifier 2 on the rectification side, to excite a core of the transformer 10 of the forward converter B in non-operating state. In the excited state of the core, if the forward converter B delays in starting switching operation, at the moment, an excessive surge voltage is generated at the main switch Q1 and the diode D1, and a problem arises that these circuit elements are damaged thereby.
Further, when the leakage inductance of the transformer 10 is large, at the moment when the main switching element Q1 is turned on, a surge voltage caused by the above-described leakage inductance is generated across the gate and source of the synchronous rectifier 2 on the rectification side, and the diode D1, causing a problem to occur that it is difficult to use lower voltage components and instead higher cost high voltage rated components must be used.